Re: [Boost-users] boost::function performance issues with C++11

Subject: Re: [Boost-users] boost::function performance issues with C++11
From: Angelo Mondaini (oangelo_at_[hidden])
Date: 2014-07-03 20:50:00

I guess that with -std=c++11 the compiler may not do all the possible
optimizations, since c++11 is "new".
How about go wild and use -O3 as compilation flag?
This will give more freedom to the compiler optimization, and maybe more
velocity.

2014-07-03 21:23 GMT-03:00 饭桶 <athrun911500_at_[hidden]>:

> Hi Michael,
>
> Thanks for your suggestion. I understand that using a reference or a
> pointer will achieve better performance, and I really appreciate you
> pointed out that for me.
> But I intend to do this to highlight my point, that *the performance are
> quite different with or without -std=c++11*. And I'm really curious about
> that.
>
> Consider a situiation, that I'm using boost::function. However, the
> performance could be seriously affected by using a compiling flag
> (-std=c++11). That will be very weired to me, unless someone can tell me
> what happens there.
>
> BTW, when I say default, I mean release mode.
> Thanks
> Xuepeng
>
>
>
>
>
>
> At 2014-07-04 07:55:39, "Michael Powell" <mwpowellhtx_at_[hidden]> wrote:
> >On Thu, Jul 3, 2014 at 4:10 PM, 饭桶 <athrun911500_at_[hidden]> wrote:
> >> Hi,
> >>
> >> Does anyone know why -std=c++11 causes so much difference on
> >> boost::function?
> >>
> >> I was planed to understand if there any performance issues with big size of
> >> parameters.
> >> So I wrote a function that takes a vector as parameter, like func2 shows. I
> >> know it's better to use a pointer or reference as function parameter. I just
> >> want to evaluate the performance, so let's keep the vector as parameter.
> >>
> >> However, I found that it's quite slow when compiled with -std=c++11. In
> >> detail, it takes 173874 milliseconds with C++11, while it takes 3676 seconds
> >> without C++11.
> >>
> >> About 50 times slower!! How can that be?
> >>
> >> In my opinion, I thought boost::function should had the same performance
> >> with std::function. So I decided to try std::function in C++11 directly,
> >> Finally, it takes about 29233 milliseconds. That's till 8 times slows!
> >>
> >> Can anyone tell me what happend here?
> >
> >I don't know the inner workings of either boost::function or
> >std::function. It's not boost's fault per se, but there are a couple
> >of things you could do differently.
> >
> >> int func2(std::vector<int> i){
> >>     total += i.size();
> >>     return i.size();
> >> }
> >
> >Pass a reference or even a pointer instead of the whole vector. You
> >are copying the vector every time.
> >
> >>     const int T = 1000000;
> >>     s = boost::chrono::system_clock::now();
> >>     for (int i = 0; i < T; ++i)
> >>         call(boost::bind(&func2, v));
> >>     e = boost::chrono::system_clock::now();
> >
> >You are also binding func2 every time; not sure if that's getting
> >optimized or not.
> >
> >You likely want to bind with a placeholder instead of the vector
> >itself, then call the binding itself. i.e.
> >
> >auto binding = boost::bind(&func2, _1);
> >binding(v);
> >
> >I do that frequently enough; and with more event-driven systems,
> >boost::signals, etc, it is unavoidable. You want the placeholder
> >instead, once-bound/later-called.
> >
> >> In case you need to know my enviorment, my OS is Arch, compiler is gcc
> >> 4.9.0, and optimizations are default.
> >
> >Default can mean a lot of things. Debug or release mode? Beyond those
> >two broad categories, do verify your settings and build for release
> >mode.
> >
> >HTH
> >
> >> The execution time (ms) of three versions I tried:
> >> boost::function with C++11 : 173874
> >> boost::function without C++11 : 3676
> >> std::function in C++11 : 29233
> >>
> >> Any thoughts are appreciated!
> >> Thanks,
> >> Athrun
> >>
> >>
> >>
> >> _______________________________________________
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>
>
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